This invention relates to engine cooling systems. More specifically, this invention relates to an improved charge air cooling system including a pair of back-to-back heat exchangers for two stage cooling of engine charge air.
A wide variety of cooling systems are known in the prior art for cooling charge air supplied to combustion engines. Typically, these cooling systems relate to one or more heat exchanger devices connected between a charge air compressor and an engine air intake manifold, such as in a turbocharged or supercharged internal combustion engine. The heat exchanger device is provided for cooling the charge air prior to supply thereof to the engine intake manifold in order to reduce the volume and to increase the density of the charge air supplied to the engine, and thereby improve engine performance and efficiency. Moreover, the heat exchanger device cools the charge air to lower overall engine heat load, and thereby reduce thermal loading to improve engine durability.
In the prior art, ambient air has been proposed for use as the cooling medium in a charge air heat exchanger. Use of ambient air is advantageous in that it comprises a readily available heat sink, and frequently comprises the lowest temperature coolant readily available in an engine environment, particularly in a motor vehicle application. See, for example, U.S. Pat. Nos. 4,059,080 and 4,062,188. However, to provide the desired degree of cooling, charge air heat exchangers using ambient air as the cooling medium have comprised relatively large heat exchangers which are not well suited for use in vehicle applications. Accordingly, with vehicles, charge air coolers using ambient air either have not been used because of their excessive size, or have been undersized so as not to provide the desired degree of cooling.
A more common prior art charge air cooling system comprises an air-to-liquid heat exchanger coupled into a liquid cooling circuit, wherein a liquid coolant is circulated through the heat exchanger in heat transfer relation with the charge air. See, for example, U.S. Pat. Nos. 3,229,456 and 4,061,187. Air-to-liquid heat exchangers are advantageous in that they provide a relatively small unit package with substantially improved cooling capacity compared with air-to-air devices, and thereby are more readily adapted for use on vehicles. However, for maximum space savings, an air-to-liquid charge air heat exchanger frequently is coupled into the engine cooling system so as to provide a single liquid flow system for cooling both the engine and the charge air. In these systems, the cooling liquid supplied to the charge air heat exchanger has a relatively high temperature level by virtue of its primary function of maintaining engine operating temperature within prescribed limits. Because of this high operating temperature level, the cooling liquid therefore generally is unable to provide the desired degree of cooling of the charge air. For examples of air-to-liquid charge air cooling systems coupled into the engine cooling system, see U.S. Pat. Nos. 3,397,684; 3,442,258; 3,863,612; and 3,872,835; British Pat. No. 920,020; German Pat. Nos. 1,140,018 and 1,223,196; and German Pub. No. 2,335,248.
Some prior art charge air cooling systems have attempted to provide improved charge air cooling by including multiple air-to-liquid heat exchangers with separate, independent cooling flow circuits for supplying liquid cooling to each heat exchanger. See, for example, U.S. Pat. Nos. 3,232,044 and 3,450,109. While such systems provide improved cooling of charge air, they also require substantial additional coolant flow circuitry and heat exchangers whereby these systems have generally not been commercially adapted for use in vehicle applications. Other prior art systems have attempted to provide improved cooling of charge air by including multiple air-to-liquid heat exchangers wherein one or more heat exchangers are coupled into the engine cooling system in series with the engine. Such systems are shown by way of example in U.S. Pat. Nos. 3,134,371 and 3,439,657; and in German Pub. No. 2,655,017, and are disadvantageous in that their cooling efficiency is highly dependent upon the amount of cooling required by the engine.
Still other multiple heat exchanger systems in the prior art comprise mechanical refrigeration systems such as those shown in U.S. Pat. Nos. 3,141,293 and 3,306,032. In these refrigeration systems, the compressed charge air is circulated through at least one evaporator or the like of a refrigeration system to provide substantially improved cooling of the compressed charge air. However, refrigeration schemes are not desirable in that substantial energy is required to operate the mechanical cooling system. In the case of vehicular engines, mechanical refrigeration systems comprise a substantial and undesirable load on an engine which detrimentally affects engine performance.
The present invention overcomes the problems and disadvantages of the prior art by providing an improved charge air cooling system including in series a pair of air-to-liquid heat exchangers for providing first and second stage cooling of the charge air prior to supply to the engine. The two stages are provided with progressively cooler liquid coolant for providing efficient, maximum cooling of the charge air prior for supply thereof to the engine.